The present invention relates to optical couplers and especially to optical couplers adapted for coupling optical fibers and providing wavelength division mutliplexing of light to a plurality of fibers which are coupled, via the optical coupler, to a source of broad band light.
Optical couplers, in accordance with the invention, are designed using lenses having anamorphic surfaces which handle light between input and output ports which are offset from the optical axis of the coupler and to reduce optical aberrations in the light reaching the output ports such that the efficiency of the coupler is enhanced the efficiency of the coupler may be a multiplicity of times better than that of couplers of conventional design having lenses with rotationally symmetric surfaces, even when such surfaces are aspherically corrected. The surfaces of the lenses in the improved coupler are preferably toroidal.
Conventional fiber optic couplers providing off axis coupling, that is from ports which are spaced from the optical axis and having wavelength division multiplexing beam splitters or filters, have used lenses which rotationally symmetric surfaces or gradient index lenses. U.S. Pat. No. 3,973,560 issued to A. Milton and U.S. Pat. No. 4,213,677 issued to S. Sugimoto et al, show the general design of such optical couplers with lenses having rotationally symmetric refracting surfaces. Such lenses have introduced aberrations and distortion of the light passing via the lenses to output ports of the coupler, thereby causing the coupler to introduce undesirable values of insertion loss. This has led to the adoption of gradient index lenses which are tolerant of off axis location of the ports even at relatively wide separations, say around 200 to 300 microns (micrometers). Since the index of refraction of the gradient index lens material is different from the silica material of the optical fibers, it is difficult to closely couple, say by fusion bonding, the fibers to the gradient index lenses of the coupler without introducing undesirable reflection which can deteriorate the insertion loss presented by the coupler and cause a high level of back reflection into the fiber.
The present invention provides a coupler using non-gradient index lenses which may be made of the same material as the optical fibers but having insertion loss comparable to couplers with gradient index lenses.
Accordingly, it is the principal object of the invention to provide improved optical couplers, especially suitable for fiber coupling, and having insertion loss better than couplers using non-gradient index lenses of conventional design.
It is a further object of the invention to provide an improved fiber optic coupling system where the fibers associated with the coupler are spaced from the optical axis of the coupler, even by hundreds of microns, and without adversely affecting the coupling efficiency, by minimizing the insertion loss introduced by the coupler.
It is a still further object of the invention to provide an improved optical coupler especially suitable for fiber optic coupling applications, which also provides wavelength division multiplexing and couples light among the ports efficiency by minimizing insertion loss due to the coupler.
It is a still further object of the present invention to provide an improved optical coupler having more than two output ports, as with an input port and two output ports on one side of the coupler and an output port on the opposite side of the coupler.
Briefly described, a system for coupling an input port providing a light source to a plurality of output ports, with the input and output ports being fiber optic ends, in accordance with the invention uses a pair of lenses which are arranged in collimating and decollimating relationship to provide light paths from the source port to the output ports. An element, such as a wavelength beam splitter as may be provided by an interference filter via which light collimated by one of the lenses passes and which divides the light into a plurality of paths to the output port, is disposed between the collimating and decollimating lenses. At least one of the lenses has a rotationally asymmetric (preferably toroidal surface) which provides for correction of aberrations in the light reaching the output ports. By virtue of the asymmetric surface, the wavelength distortion and consequent insertion loss is much better than with conventional fiber optic couplers.